The present invention relates to an improvement of cross-roll forging machines, and more particularly of the tool thereof.
The conventional drop hammer and press forging processes have some common problems. Among these are relatively low productivity because of the after-treatment required for removing burrs from forgings, buffing them, etc., and also the large vibrations and noise which are produced during such operations. In order to overcome these problems, in recent years there have been devised cross-roll forging machines in which a pair of forging tools are pressed against a rotating bar and displaced in a direction at right angles to the axis of the rotation of the bar, whereby the bar is drawn and shaped. A cross-roll forging machine has the advantages that the vibrations and noise may be considerably suppressed, the forgings have no burr and have improved strength, waste of material may be minimized, and the automation of forging process is feasible, thus increasing production. Thus remarkable advances have been made in forging techniques by the introduction of cross-roll forging machines.
However, there has been a problem that materials to be forged by cross-roll forging machines are limited because the materials to be forged must be heated in a furnace to a suitable forging temperature prior to the forging process. With steel materials which have high forging temperatures and whose forging temperature range is relatively wide, the rotary forging machines exhibit excellent forging capabilities. On the other hand, in case of light metal alloys such as aluminum alloys, copper alloys, stainless steels and the like with relatively low forging temperatures and with a relatively narrow range of forging temperatures, heat is transmitted from the workpiece to the tools during the forging process so that the temperature of the workpiece drops below its forging temperature. Consequently, it becomes impossible to attain forgings with high quality. To overcome this problem, it has been proposed to heat the forging tools with burners, but this burner heating method is disadvantageous in that the efficiency of forging operations inevitably drops, and soot attaches to the tools, thus resulting in surface defects of the forgings. Also, accurate temperature control is impossible.
Because of these drawbacks, the only alternative has been to use the drop hammer and press forging machines to forge blank materials except steel materials. However, when the temperature of the workpiece drops below the desired forging temperature during the forging process, the workpiece must be removed from the forging machine and recharged into a furnace to be heated again to be desired forging temperature, and then placed again into the forging machine. Such forging steps are laborious and inefficient.
For the sake of explanation, the forging temperatures of some materials are shown in Table I.
Table I ______________________________________ Range of temperatures at which Materials workpieces may be forged (.degree. C) ______________________________________ Steels 1,200 to 1,000 Aluminum alloys 420 to 400 Stainless steel 1,050 to 950 ______________________________________
From Table I it is seen that the forging temperatures of steel materials may vary over a relatively wide range. Therefore, during a forging operation with a cross-roll forging machine, the temperature of a steel workpiece may drop within the range from the highest temperature immediately after discharge from the furnace to the lowest allowable forging temperature so that the workpiece may be satisfactorily forged. On the other hand, light metal alloys and the like have low forging temperatures and a critically narrow forging temperature range so that the temperature of a workpiece drops below the lowest allowable forging temperature before the forging operation is completed. As a result, the forgings have poor qualities. Even when the forging tools are heated with burners during the forging operation, the forging efficiency drops, the maintenance of the tool at desired temperature is extremely difficult, and soot attaches to the tools, causing surface defects of the finished workpieces. Therefore the method of heating the forging tools with burners cannot be employed in the process for forging materials with the critical forging temperatures.